1. Field of the Invention
The present invention relates to energetic polymers particularly useful in rocket propellant binder compositions and formed from polyethers bearing pendant azide groups cross-linked without a catalyst by a diacetylene.
2. Description of the Prior Art
Composite solid rocket propellants are presently manufactured using a variety of liquid di- and tri-functional polyol prepolymers which can be cross-linked to form elastomeric polyurethane binders which are used to form composite solid rocket fuel grains having superior mechanical properties. The polyurethane binders are widely used in both propellants and plastic bonded explosives and were developed during the 1950's to take advantage of the longchain polyalcohols which were becoming available in a wide molecular weight range. These polyalcohols, when reacted with diisocyanates form stable polyurethane polymers which could be used in large, case-bonded rocket motors. Even today, the most versatile binder systems for compounding composite propellants are derived from the reaction of hydroxyl-terminated polyols with diisocyanate to form a polyurethane network. Representative polymers are disclosed in U.S. Pat. Nos. 3,132,976; 3,245,849; and 3,350,245.
The polyurethane binders in composite propellants have excellent mechanical properties. Furthermore, their shrink free, low-temperature controlled cure leads to good reproducibility and high reliability of the resulting propellants. The resulting propellants typically demonstrate better processing characteristics and better aging characteristics than the previously used epoxy cured carboxylated binders. In addition, the polyurethane binders are known to have acceptable compatibility with metal powders and oxides.
However, there are unresolved drawbacks to these particular polyurethane rocket propellants. For example and for reasons not entirely understood, it is difficult to obtain adequate urethane cross-linking in high-energy propellants which require high levels of nitrato plasticizers. This problem may have something to do with the fact that isocyanates homopolymerize as well as react with un-nitrated hydroxyl groups that may be present within the plasticizers.
Another drawback is that upon aging, an undesirable side reaction between urethane and nitrate ester plasticizer occurs which degrades the mechanical properties of these propellants in which a nitroso derivative is formed which decomposes to rupture the polymer chain while liberating nitrogen gas causing cracking of the propellant grain. This process often limits the service life of nitrate ester containing propellants.
Also, the curing of hydroxyl-terminated prepolymers with diisocyanates, to form the polyurethane binders, requires particular catalysts such as organometallic compounds. Although these catalysts can reduce some of the undesirable side reactions, this reduction of the undesirable side reactions is not complete. It has been found that, above ambient temperatures, the presence of residual catalysts from the urethane cure causes a random reversal of the urethane cure reaction and partial depolymerization in the urethane linkages forming isocyanate and hydroxyl-terminated polyether. Once formed, the isocyanate groups can react with themselves to produce, for example, isocyanurates, compounds having a reduced isocyanate content. This loss of isocyanate prevents the urethane linkages from completely regenerating. Consequentially, the binder gradually loses its elasticity and the mechanical properties become unacceptable.
High energy nitrate ester plasticized polyurethane propellants tend to have low values of tensile stress and modulus. This is particularly a problem with highly plasticized azido and nitrato polyoxetanes. These energetic polymers have sterically hindered hydroxyl groups which are slow to react with isocyanates and may not form a complete polymer network. Nitrocellulose has been added to enhance these properties, but it tends to degrade the elongation and thereby decrease toughness.
Accordingly, it would be a substantial advancement in the art to provide polymers useful as a high-energy solid rocket propellant binder from hydroxyl-terminated prepolymers, particularly those having azide groups, if these polymers are characterized by the advantageous properties of polyurethanes, but do not have the above described drawbacks.
U.S. Pat. No. 3,645,917 discloses in Example 1 a linear polymer made by subjecting an epichlorohydrin-ethylene oxide polymer to sodium azide resulting in a polymer with 15.5% of the repeating units corresponding to those of the glycidyl azide polymer (GAP) discussed below. In Example 2 a similar polymer having only 3.9% of such repeating units is produced and cross linked in Example 7 by p-diethnylbenzene in a proportion of one ethynl group per azide group so that it is apparent that substantially all of the original azide groups are converted to triazole linking groups forming a highly cross linked polymer insoluble in acetone and benzene.
This polymer is inherently deficient as an energetic and elastomeric binder for four reasons. First, the polymer provides little energy because the triazole groups individually provide much less energy than the original azide groups and because, in any event, only a small percentage of the repeating units have even a triazole group. Second, the polymer is not soluble in energetic plasticizers such as butanetrioltrinitrate (BTTN) and energetic fillers, such as cyclotetramethylenetetranitramine (HMX), cannot be added. Third, the highly linked sidechains result in a relatively rigid polymer having little elongation before breaking. And fourth, the substantial number of unreacted chlorines from the epichlohydrin would result in low stability of compositions including this polymer.